Irritant contact dermatitis is caused by the direct effect of a chemical or irritant on the skin. It occurs in areas where the skin is not well protected by hair, such as the feet, chin, nose, hocks, stifles, and the undersurface of the body, including the scrotum. Irritant contact dermatitis can occur after a single exposure or repeated exposure.
Irritant contact dermatitis produces itchy red bumps and inflammation of the skin. You may notice moist, weepy spots, blisters, and crusts. The skin becomes rough and scaly and hair is lost. Excessive scratching damages the skin and sets the stage for secondary pyoderma.
Chemicals that can cause irritant dermatitis include acids and alkalis, detergents, solvents, soaps, and petroleum byproducts.
Less commonly, the skin becomes sensitized to a certain chemical and a delayed type of hypersensitivity reaction develops. This is allergic contact dermatitis. This rash is indistinguishable from that of irritant contact dermatitis, but appears after repeated exposure and often spreads beyond the site of contact.
Allergic contact dermatitis can be caused by chemicals found in soaps, flea collars, shampoos, wool and synthetic fibers, leather, plastic and rubber dishes, grasses and pollens, insecticides, petrolatum, paint, carpet dyes, and rubber and wood preservatives. Chemicals used to clean your carpet are a frequent source of irritation. Neomycin, found in many topical medications, can produce allergic contact dermatitis, as can other drugs and medications.
Plastic and rubber food dish dermatitis affects the nose and lips (see Plastic Dish Nasal Dermatitis).
Flea collar dermatitis is an allergic contact dermatitis. The signs include itching, redness, hair loss, and the development of excoriations, scabs, and crusts beneath the collar. It can be prevented to some degree by airing the collar for 24 hours and applying the collar loosely. The flea collar should fit so that you can get at least two fingers beneath the ring. However, if your dog develops flea collar dermatitis, the best course of action is not to use a flea collar.
Treatment: Consider the area of involvement and identify the chemical or skin allergen causing the problem. Prevent further exposure. Treat infected skin with a topical antibiotic ointment such as triple antibiotic. Topical and oral corticosteroids prescribed by your veterinarian can relieve itching and inflammation.